The present invention relates generally to a centrifugal blasting machine, and more particularly, to an improved blasting wheel assembly and structure for feeding abrasive to the wheel assembly.
Centrifugal blasting wheels usually consist of a rotatably supported wheel assembly having circumferentially spaced apart impeller blades extending radially with respect to the axis of rotation of the wheel assembly. Each impeller blade has an outer end that terminates adjacent the periphery of the wheel and an inner end that is spaced radially from the axis of rotation of the wheel. The inner ends of the impeller blades define a central abrasive supply space into which abrasive, usually particulate shot, is fed for discharge onto the impeller blades.
Generally, two methods are employed for feeding abrasive onto the impeller blades. The first method uses a rotatable impeller unit positioned in the abrasive supply space for propelling abrasive onto the inner ends of the impeller blades. Such an impeller unit is disclosed in U.S. Pat. No. 3,290,827 wherein a mechanically driven impeller throws the shot onto the impeller blades. The significant problem with an impeller of this type is that it wears resulting in a changing feed of shot to the impeller blades which in turn results in an undesirable blast pattern on the work. Consequently, hot spots and over-blast problems occur early in the life of the blasting machine.
The second method of feeding abrasive onto the impeller blades uses a pneumatic feed system, such as the feed system disclosed in U.S. Pat. No. 2,590,576. The pnuematic feed system of this type uses pressurized air to propel the abrasive onto the impeller blades and usually includes a tubular member having a main portion whose axis is coaxial with the axis of rotation of the wheel assembly and a discharge extension extending radially outwardly with respect to the main portion and having a discharge opening terminating adjacent to and in close proximity to the inner ends of the blades. Abrasive is gravity fed to the tubular member and pressurized air is supplied to the tubular member to propel the abrasive through the tubular member onto the impeller blades. Because the main portion of the tubular member is aligned with the axis of rotation of the wheel assembly, a substantial amount of energy must be expended in imparting a sufficient initial velocity to the abrasive to propel it onto the impeller blades. Also, conventional impeller blades as disclosed in U.S. Pat. No. 2,590,576 are relatively long so that control of the abrasive as it moves along the impeller blade is reduced thereby reducing the control of the blast pattern which sometimes results in less than an optimal blast pattern.
It is the object of the present invention, therefore, to provide an improved centrifugal blasting machine characterized by relatively short impeller blades and a pneumatic feed system requiring reduced energy to propel the abrasive onto the impeller blades.